1. Field of the Invention
This invention relates to super-abrasive tools for grinding and polishing the curved surface of optical and ophthalmic lenses, including toric surfaces, and to a novel method of manufacturing the tool. More specifically, it relates to a novel super-abrasive grinding and polishing tool and method of making the same which increases production, reduces costs and produces an improved product.
While the present invention will be described with particular reference to several embodiments of the novel tool and method of manufacturing the same, particularly as to its application as a toric generator, it should be understood that the invention is not limited thereto. The concepts set forth herein can be readily adapted for use in connection with other grinding, polishing and lapping operations and the like, as those skilled in the art will recognize in the light of the present disclosure.
As used herein, the term "super-abrasive" refers to abrasive media suitable for grinding and polishing conventional ophthalmic and optical lens glass and having a hardness on the Knoop scale in excess of about 3000 kg/mm.sup.2. A comparison of Knoop and Mohs hardness values for conversion purposes is available in standard handbooks. Conventional super-abrasives include natural and synthetic industrial diamonds and cubic boron nitride, although the present invention is not necessarily limited thereto.
2. Description of the Prior Art
As those skilled in the art appreciate, many optical and ophthalmic lenses have one curvature on one axis but a different curvature on the axis at right angles thereto, e.g., a toric lens. In a typical prior-art grinding operation, such toric surfaces are generated by a three-step process, i.e., a rough grinding step, a fine grinding step and a polishing step.
In the first or rough grinding step the glass blanks are ground to the general shape of the desired finished product, e.g., a ellipsoid configuration as a first approximation, using diamond-impregnated peripheral or ring-type grinding wheels of conventional design. In the second or fine grinding step a hard metal tool having the desired toric surface is reciprocated across the roughly-ground concave or convex ellipsoid lens surface in one direction to provide the desired curvature in that direction and then at right angles thereto to get the other curvature. In this step the grinding action is provided by means of a liquid slurry containing loose common abrasive particles in finely-divided form, e.g., garnet, silicon carbide, emery, alumina or the like, which slurry is continuously poured over the reciprocating tool and lens blank as the operation proceeds. In the third or polishing step a polishing pad is applied to a hard metal tool and the reciprocating action is repeated. The polishing action is provided by a liquid slurry containing loose abrasives having the desired polishing characteristics, e.g., zirconium oxide, cerium oxide or the like.
The second or fine grinding step is critical, as those skilled in the art recognize. For example, any substantial improvement therein, particularly from a quality standpoint, has the compounded benefit of simplifying and decreasing the cost of the subsequent polishing step. It is to this critical second step that the present invention is primarily directed. The third or polishing step also has been improved by extension of certain concepts of the invention to a novel polishing pad.
It would be particularly advantageous in the second step to use a fixed-super-abrasive concave or convex grinding medium having the desired toric configuration to grind the desired surface, rather than a loose abrasive slurry. This would speed up the operation, reduce its cost, and has the potential of improving quality. It would otherwise avoid the many disadvantages of handling a loose abrasive slurry. It would also minimize an annoying slurry disposal problem, particularly from an ecological standpoint. It would also avoid the substantial cost of having to periodically replace, refinish, or otherwise restore the curvature to, the hard metal tool which is simultaneously subjected to the abrasiveness of the slurry.
A hard metal tool, e.g., steel, can, of course, be readily shaped to the desired toric configuration using, for example, a single surface tool such as a pointed diamond impregnated tool. It was heretofore impractical, however, to attempt to generate a large-area, diamond-impregnated, toric-configured surface because of the rapid wear in a single surface tool due to the presence of the diamond particles. Use of diamond impregnated tools for fine grinding has been possible for producing spherical lenses, which have only a single curvature and can be ground by a tool simply rotating about a single axis perpendicular to the lens surface. The inherently more complex toric lenses required more complex movements of both tool and lens and prevented introduction of the diamond impregnated tools. Accordingly, much of the industry has continued with the time-consuming, costly and cumbersome loose abrasive approach.
One prior-art approach to coping with certain of the problems associated with the loose-slurry grinding and polishing techniques is to apply protective pads to the hard metal tools having the desired toric configuration. The pad experiences most of the undesired wear from the abrasive slurry; and the protective pad is periodically replaced before substantial wear of the toric surface of the hard metal tool occurs. Such approach is represented, for example, in a number of prior-art patents.
Beasley U.S. Pat. No. 3,594,963, for example, discloses the use of a replaceable grinding pad for a lens grinding tool. The pad may comprise a stamped sheet steel blank having a thickness between 0.005 and 0.010 inch, which is adhered to the grinding tool by means of a suitable contact cement of adequate bonding strength.
Faas U.S. Pat. No. 3,144,737 and Sarofeen U.S. Pat. No. 3,522,680 similarly discloses grinding pads or overlays which conform to the shape of the tool and protect it from the grinding action of the abrasive slurry. But such approaches, while advantageous, represent only a partial solution to what is otherwise a high-cost, slow, cumbersome method of manufacture.
The aforementioned problems are further compounded by the very large number of spherics and toric encountered in conventional grinding and polishing operations. This is reflected in high inventory requirements and/or extended delays in filling orders.